The use of energy absorbing motor mount assemblies to attempt to insulate or reduce the transmission of vibrations from the engine of the vehicle to the frame and passengers is widespread. Conventionally a vehicle engine will be supported from the frame by three or four motor mount assemblies which include, as a part thereof, a rubber block or washer that reduces vibration and shock loads.
In recent years a second function of engine motor mounts has become quite important. In addition to damping vibration, shock loads and engine roll, the engine motor mount assemblies can also act as a significant factor in enhancing the safety of a vehicle during collision conditions. When a motor vehicle impacts a relatively immovable object, at even moderate speeds, it is not uncommon for the engine to be driven rearwardly through the fire wall of the vehicle and into the passenger compartment. This often occurs as a result of the engine motor mount assemblies being unable to absorb the impact energy.
Some attempts have been made to provide engine motor mounts which are capable of absorbing energy during the shock loading of a collision. Typical of these prior efforts are the apparatus disclosed in U.S. Pat. Nos. 3,869,017, 3,851,722, 3,815,703 and 3,718,304. In the first of these U.S. Pat. No. 3,869,017, a system is disclosed in which the vehicle frame is formed to provide an energy absorbing capability and the motor mounts for the engine are break-away mounts formed to fracture and enable the engine to separate from the frame. While this approach may be desirable for certain types of collisions, particularly when the frame or the body of the vehicle is stopped but the engine is not, it has certain drawbacks. If the engine is stopped by the impact of collision before the frame is, the engine will be sheared away from the frame while the car body continues forwardly with the result that the fire wall may be driven over the engine. A similar approach is employed in U.S. Pat. No. 3,851,722.
The system of U.S. Pat. No. 3,815,703 is also similar in that the engine is rigidly secured to impact bars so that it will be immediately stopped upon a collision. The energy absorbing capability of the system is provided between the impact bars and the frame and in the frame itself. Once the engine is stopped, it is easier to decelerate the frame and its lowered moment of inertia. There is always the possibility, however, that the frame will not be decelerated to zero velocity before the passenger compartment of the vehicle is driven over the engine.
In U.S. Pat. No. 3,718,304 an engine motor mount assembly is disclosed in which rubber blocks are employed as an energy absorbing means in combination with shear pins which are designed to shear upon impact of the vehicle with an immovable object. Under a high energy impact in which the shear pins are severed, the rubber block is also loaded in shear, as it absorbs energy. This approach, however, is dependent in no small part upon the strength of the rubber block in shear and its bonding to the respective movable parts. This assembly does not provide a positive limit on the movement of the mounting brackets once the shear pins have been severed.